Intersections

Intersections are a key part of the urban landscape and, therefore, should be treated properly to maximise attractiveness, safety and efficiency. Intersection design has undergone a fundamental shift over the past decades. What was once seen as simply an exercise in processing the highest number of vehicles has now been recast as an exercise in safety.

Intersections, by definition, are points of conflict. Experience tells us that the best way to minimise the outcomes of those conflicts is through speed management—not by assigning priority as is traditionally done through traffic control devices. The quality of an intersection environment can vary significantly depending on turning radii, the presence of refuge islands, the continuity of cycle tracks, and other design features.

Intersections, rather than the standard section of a street, are the limiting factor in vehicle capacity. Therefore, intersection design needs to take into account the impact of design choices on mobility. However, this emphasis on mobility should not be confused with an emphasis on private motorised traffic. Instead, it may be desirable to design an intersection in such a way that prioritises throughput of public transport, cycles, and pedestrians.

Intersection typologies

Intersections can take a variety of forms depending on the level of pedestrian activity, bicycle traffic, vehicle volume, presence of BRT, and street cross sections:

  • Signalised intersections are the preferred configuration for urban intersections of major streets with large volumes of pedestrians and cyclists. Signals manage movements on all legs of the intersection, providing crossing opportunities for pedestrians and cyclists.
  • Roundabouts improve safety for vehicles by simplifying interactions at unsignalised intersections. However, they present challenges for pedestrians and cyclists because they increase the size of intersections and divert NMT movements from their desire lines. Roundabouts are warranted at locations with moderate traffic volumes: up to 15,000 annual average daily traffic (AADT) for a mini roundabout, 25,000 AADT for a single-lane roundabout, and 45,000 AADT for a two-lane roundabout. With higher volumes, roundabouts should be converted to compact signalised intersections without the central traffic circle to reduce delay for motor vehicle and NMT users alike. Roundabouts are acceptable in intersection retrofits but are not the preferred layout for new streets.
  • Squareabouts are a means of managing right-turning traffic at large intersections while minimising signal cycle times. Squareabouts make the right-turn phase obsolete by creating left- turn queuing space within the intersection itself. Vehicles queue in this space during one phase and exit during the next phase. Squareabouts are a valuable option on BRT corridors. While BRT would require the addition of extra phases to a typical four-phase intersection, the squareabout accommodates all turning movements in only two phases.
  • Stop-controlled intersections are appropriate for smaller intersections with low to moderate traffic volumes. Stop lines should be provided.
  • Mini roundabouts are the safest type of intersection on smaller streets.
  • Grade separations may be warranted if they prioritise NMT and public transport movement. All grade-separated facilities along planned BRT corridors should incorporate provision for BRT lanes and BRT turning movements at intersections.

Intersection control

Intersection control mechanisms should facilitate safe transitions between the intersecting routes, which may have different traffic speeds. There are several possible sequences of signal phases for a given intersection. The optimal phasing design is determined by the relative volumes of the various movements taking place at an intersection. The physical layout of an intersection must be designed in conjunction with the signal phasing.

The simplification of signal cycles through the elimination of turning movements can help reduce delay at intersections, particularly along BRT corridors. As described later in this section, squareabouts combine straight and turning movements, allowing for a two-phase cycle. Signal cycles also can be simplified through changes at the network level. For example, a left turn can be substituted by three right turns.

Design standards

  • Signal timing should be optimised according to traffic volumes on different legs of the intersection. Pedestrians should be given priority, followed by bicyclists, public transport, and other vehicles in that order. Streets climbing up slopes shall be given priority over other legs of an intersection.
  • Provide leading pedestrian signals allowing pedestrians to begin crossing before the signals for vehicles along the same leg turn green.
  • The minimum phase duration is determined by the time pedestrians need to cross the street, assuming a walking speed of 1 m/s.
  • Adopt two-phase signal cycles and intersection designs along BRT corridors. Intersections where BRT corridors pass through roundabouts should be signalised.
  • Phasing sequences ensure that the final vehicles from each phase are in a different part of the junction from the starting vehicles in the next phase. For example, for four straight-plus-left phases, a clockwise sequence is preferred.
  • Signalised intersections should be fitted with audible pedestrian signals in at least one local language and English.
  • Traffic lights shall be located at both sides of the lanes entering the intersection, and also in the median on the opposite side of the intersection, as applicable

Crosswalks

Crosswalks delineate an area that is reserved for pedestrian movement while perpendicular traffic is stopped. Crossings should only be marked where vehicles are required to stop, such as at signalled intersections. At unsignalised intersections, painted crosswalks do not necessarily improve pedestrian safety unless accompanied by a physical measure such as a speed bump or speed table.

Design standards

  • Zebra crossings on all legs of all signalised intersections.
  • 5 m wide pedestrian crossings (3 m minimum) and 2 m wide cycle track crossings.
  • Pedestrian crossings aligned with desire lines. People will cross the street using the shortest route, so it is best to accommodate movements along desire lines. Align crossing elements (zebra crossings, median refuges, and kerb ramps) in a straight line and maintain the same width.
  • Raised crosswalks (tabletop crossings) at +150 mm at unsignalised zebra crossings. This applies to crossings of smaller streets along a corridor, slip lanes, and elsewhere. Signalised crossings may be raised as well. The entire intersection may be raised.
  • Provide bicycle facilities through intersections. Protect cyclists from drivers, especially turning drivers. Direct cyclists through pedestrian areas. Include slow, shared zones where modes and directions interact.
An intersection should be sized to minimise pedestrian crossing distances while accommodating left turns of a design vehicle.

Turning radius

The concept of the turning radius is relevant in the context of designing street corners and left turn pockets. Larger vehicles require more space in order to take a turn, so intersection designs need to take into account the size of vehicles that are expected to pass through an intersection.

Since larger turning radii encourage faster vehicle speeds, tighter corners are preferred because they improve safety for pedestrians and cyclists. For local streets that cater to light vehicles, as well as intersections of major streets with local streets, a 4 m kerb radius is appropriate.

While larger streets need to take into account the turning radius requirements of buses and trucks, it should be noted that the effective turning radius is often much larger than the radius of the built kerb. The design of the kerb should assume that trucks and buses make the largest turn possible. The following table shows the recommended kerb radii (m) by intersection typology. If the exit leg has more than two lanes, a smaller kerb radius is possible because the sweeping path of a large vehicle can pass through the available lanes.

PAS

SAS

CS

LS

PAS

8.0

SAS

8.0

6.5

CS

6.5

6.5

4.0

LS

4.0

4.0

4.0

≤ 2.0

Refuge islands, medians, and bollards

Pedestrian refuge islands separate conflicts, so pedestrians can judge whether it is safe to cross by looking at and analysing fewer travel lanes and directions of traffic at a time. Tall, bushy plants should be avoided in medians because they obstruct pedestrian visibility. In the case of triangular islands adjacent to free left turn lanes, the island must remain free of landscaping and fencing in order to serve as a refuge for pedestrians.

Design standards

  • Pedestrian refuge islands separate conflicts, so pedestrians can judge whether it is safe to cross by looking at and analysing fewer travel lanes and directions of traffic at a time. Provide refuge islands where there are more than three lanes total to cross.
  • “Median tips” where there is a zebra crossing and a median at an intersection.
  • Stop lines perpendicular to the travel lane and set back at least 3 m from the zebra crossings.
  • Tall, bushy plants should be avoided immediately adjacent to refuge islands because they obstruct pedestrian visibility. In the case of triangular islands adjacent to free right turn lanes, the island must remain free of landscaping and fencing in order to serve as a refuge for pedestrians.
  • At least 1 m wide and 3 m long, matching the width of the corresponding crosswalk.

Right turn pockets

Right turn pockets can increase junction capacity by allowing vehicles to make free right turns. However, if not designed appropriately, they can compromise pedestrian safety. Traditionally, right turn lanes have been designed with a circular geometry. However, such a design is unsafe for pedestrians because it allows for fast vehicle movements. The preferred design incorporates a 30° angle of approach. Since vehicles enter the outgoing arm at a more abrupt angle, they are compelled to reduce their speeds.

The design should assume that a large vehicle completes the turn in the outermost lane of the exit arm but may enter the central lane while completing the turn. Otherwise, the right turn pocket becomes so large that smaller vehicles are able to travel at full speed around the corner. Right turn pockets can be used where the free right turns are crucial, but should be deemed an exceptional measure since pedestrian movements are not as direct in this layout.

An intersection should be sized to minimise pedestrian crossing distances while accommodating left turns of a design vehicle.

Intersections with bus rapid transit

BRT intersections should be designed to operate with two-phase signal cycles, reducing delays for buses and mixed traffic alike. Turns along BRT corridors can be managed through the following approaches:

  • Two-phase signal cycles combine straight-bound BRT and mixed traffic movement. Left turns are accomplished through the network (e.g., three right turns).
  • Signalised U-turns allow vehicles to reach the opposite side of the corridor. They also accommodate right turning vehicles (e.g., a right turn plus a U-turn).
  • Squareabouts make the left-turn phase obsolete by creating left-turn queuing space within the intersection itself. Vehicles queue in this space during one phase and exit during the next phase. BRT buses merge with the mixed traffic when moving around the central island.

Design standards

  • Adopt two-phase signals at BRT intersections. Eliminate mixed traffic left turns across the BRT lanes. Additional phases may be provided where BRT buses themselves need to turn.
  • In squareabout intersections, size the queueing space per expected turn volumes. Use corner radii of 8 m for the central island.
  • Position U-turn lanes on the outer side of the carriageway to improve visibility of turning vehicles for bus drivers and allow for an adequate U-turn radius.

Roundabouts

Roundabouts can improve safety for vehicles at unsignalised intersections with moderate vehicle volumes. They are common in locations with historical landmarks. Roundabouts increase walking and cycling distances and should be avoided where NMT volumes are high. Roundabouts need to include safety elements for pedestrians and cyclists. The cycle track should be protected from motorised traffic in the roundabout through the provision of a buffer zone. Since roundabouts are unsignalised, pedestrian and cycle crossings should be raised to the level of the footpath, with ramps for vehicles.

Design standards

  • Streets should be laid out to intersect at right angles.
  • Inner circle radius as large as possible.
  • No more than two approach lanes per direction. If there are three or more lanes, use a signalised intersection design. The number of circulating lanes should be equal to the number of lanes per direction on the largest carriageway of the intersecting legs.
  • Adopt a maximum of 2 lanes circulating the roundabout.
  • Circulating lane width of 4 m.
  • 5 m offset between the circulating lanes and the crossing to allow a vehicle to stop for pedestrians and cyclists on the exit leg without blocking the roundabout.
  • The maximum intersection grade allowable for the vehicle lanes rotating around the island is 2 percent.
  • Raised pedestrian and cycle crossings at +150 mm.

Roundabouts can be applied in areas where the volume of pedestrians is low and where there is sufficient space to accommodate vehicle and NMT elements in the roundabout design.

 

Grade separation

In general, urban intersections should be at grade in order to avoid disrupting the urban fabric, public space, and socioeconomic activities. Grade separation is not justified solely to facilitate vehicle flow, as it can lead to increased traffic and higher speeds on the overpass, which in turn can decrease safety.

However, there are cases where grade separation may be justified, such as in the construction of a BRT-only flyover to prioritize BRT buses, an NMT-only bridge or tunnel to connect parks and greenways, or in situations where there are already changes in elevation that pedestrians and cyclists are accustomed to navigating.

Design standards

  • The clear height under the grade separator shall be at least 5.5 m.

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